This invention relates to composite material suitable for physically supporting and electrically insulating bare electrical conductors, and it relates more particularly to a laminated member characterized by sufficiently high thermal rating to be substitutable for an asbestos board.
In recent years persons skilled in the art of electrical insulation have been searching for substitutes for asbestos. For example, U.S. Pat. No. 4,316,172 discloses an asbestos-free frame for a ribbon-like current conducting element that is used in the dynamic braking or retarding function of electric traction motors on railroad locomotives. During dynamic braking of a locomotive, its traction motors operate in a generating mode and supply current to resistor grids where the electrical energy is converted to heat and dissipated to the atmosphere with the aid of an associated cooling fan. According to the above-cited patent, the frame of the dynamic braking resistor grid can be made of insulator blocks formed of a hot-molded organic material. However, such material does not have a good a thermal rating as asbestos, and therefore the hot current-conducting element of the grid needs to be spaced from the insulator blocks by appropriate support clips. Even so, the frame can be damaged or, in the worst case, ignited if the element overheats and melts down due to a malfunction that causes persistent overcurrent or a blockage of cooling air. For this reason the prior art hot-molded organic material is not per se desirable for such frames.
Refractory material such as alumina silicate has a good fire retardant characteristic but not the required flexural strength for a successful dynamic braking resistor grid frame. It has been suggested that a thin layer or sheath of such material can be used as a coating on glass fiber cloth, thereby forming a composite liner that can be attached, by means of glue or cement, to the surface of a panel or duct where it serves as a fire barrier. See British patent application 2,151,544A published July 24, 1985. To provide greater physical strength and to eliminate the separate lyer of adhesive, this British application teaches a method of bonding the alumina silicate-coated glass fiber cloth directly to a base layer of epoxy resin-impregnated non-metallic fibers to form a rigid 3-layer laminate suitable for use as the exhaust duct of a gas turbine engine.
In the art of making electrical bushings, wherein a metal stud is coated with adhesive material and encircled by a molded polyester body, it has heretofore been suggested that the bonding and fluid-sealing effects of the adhesive layer between the stud and its surrounding insulator would be enhanced if the stud were etched, scored, shot-blasted, grooved, or knurled to create irregularities in its surface.